Adding and subtraction rational expression 1.5

This includes a rational expression object that should make later exercises (and the older ones) easier to create. It's a bit like polynomial one, but more flexible as it can deal with multiple variables.

It also has a function to generate regex to capture an expression. If you are silly enough to try to use this on an expression with ten terms then it will attempt to generate a regex to capture all 10! possible arrangements. So don't.

It might be better to create a validator that converts a user answer to a rational expression object and can then compare them term by term.

http://sandcastle.khanacademy.org/media/castles/petercollingridge:rat-exp/exercises/adding_and_subtracting_rational_expressions_1.5.html